1. Field of Invention
This invention pertains to a method for creating a volume of controlled atmosphere without the use of a full enclosure, and more particularly, to a system which creates a controlled gas atmosphere under any of the microscope objectives mounted on a standard microscope nosepiece. Any of the objectives can then be used to deliver laser radiation of appropriate wavelength and beam shape. The system can be used to photolytically and/or pyrolytically decompose suitable gaseous materials contained in the gas atmosphere or photoactivate a surface in the microscope objective focus plane to modify the reaction rates between the surface and the surrounding gas. This method allows for exclusion of air from the gas atmosphere. Also described is a specific source of gaseous atmosphere allowing repairs of clear defects in photolithographic masks used for making electronic integrated circuits. Also described is a method of preventing objective damage, in photolytic processes, due to material deposition on the lens or other transparent or reflective surfaces.
2. Description of the Prior Art
Laser initiated reactions, leading to localized deposition of particular metal or semiconductor substances on top of various substrates or leading to localized etching of surfaces, are becoming more and more widely used in the semiconductor industry. See, for recent reviews, "Laser generated microstructures" by Y. Ritz-Froidevaux et al, Applied Physics A, vol. 37, pp. 121-138, 1985, "Laser-induced chemical vapor deposition" by Raj Solanki et al, Solid State Technology, pp. 220-227, June 1985, "A review of laser-microchemical processing" by D. J. Ehrlich et al, J. Vacuum Science and Technology B, vol. 1, pp. 969-984, 1983. These reactions require specific atmospheric conditions to produce the desired results. Vacuum tight cells, with their associated heat- and fume-producing and noisy vacuum pumps are typically used to facilitate atmospheric control (See for example U.S. Pat. No. 4,340,617, U.S. Pat. No. 4,451,503, U.S. Pat. No. 4,525,379, and U.S. Pat. No. 4,465,529). Proper gas mixes are introduced into the cells after evacuation using either a flow-through or sealed-off mode of operation. Laser radiation, introduced through a focusing optical system and the cell window, is then typically used to activate the process by decomposing some components of the gas mixture in the gas phase or after adsorption on the surface. The products of decomposition then create deposits or etch effects. Substrate activation effects, which make the locally activated region preferentially susceptible to etch effects by the surrounding gas phase, can also be produced by laser irradiation. Spatial resolution achievable with a given process is typically limited by the fact that the microscope focusing objective has to be able to accommodate cell window thickness. This large working distance usually reduces the numerical aperture (N.A.) to a lower value than might be desirable.
Photolithographic mask repair, an application in the semiconductor industry, is addressed in U.S. Pat. No. 4,543,270 and "One-step repair of transparent defects in hard-surface photolithographic masks via laser photodeposition" by D. J. Ehrlich et al, IEEE Electron Device Letters, vol. EDL-1, pp. 101-103, 1980. In both of these references, specific gas cells are described. Ehrlich et al also give a possible approach to minimize on-the-window deposits when a photolytic repair process is used. Their approach uses a highly-convergent laser beam with much lower power density on the window resulting in a reduced deposition rate there. This method delays but does not eliminate deterioration of window transmission due to the photolytic deposition process.